Problem: Which of these numbers is prime? ${46,\ 49,\ 57,\ 73,\ 77}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 46 are 1, 2, 23, and 46. The factors of 49 are 1, 7, and 49. The factors of 57 are 1, 3, 19, and 57. The factors of 73 are 1 and 73. The factors of 77 are 1, 7, 11, and 77. Thus, 73 is a prime number.